Inductively coupled plasmas (ICPs) are advantageously used in the manufacture of devices such as integrated circuits, micromechanical devices, flat panel displays, and others. Inductive coupling is often preferred over capacitive coupling because the current flow in an inductive plasma is driven by an electromotive force with no associated scalar voltage differences. Capacitive coupling, on the other hand, can increase plasma potential, thereby causing undesirable parasitic currents and/or discharges between the plasma and various surfaces within the chamber. Relatively large pontentials can occur, causing ions to bombard surfaces at high energy, thereby sputtering the surfaces and/or injecting sputter contamination into the process chamber.
Generally, ICPs for processing are maintained within a plasma processing apparatus comprising an applicator (often referred to as an antenna) which couples high frequency electromagnetic energy through a large dielectric window of a processing chamber. In some applications the applicator is a single coil. The dielectric window is generally relatively low loss material such as quartz, alumina, or another ceramic.
Plasma processing is often performed at relatively low pressure. For example, a preselected operating pressure for plasma etching and/or plasma assisted chemical vapor deposition can be in the range of 0.1 millitorr to 100 Torr, depending on the application. However pressures outside of this range are also operable in some applications.
ICP processing apparatus often has a dielectric window spanning an upper surface of a processing chamber. Electromagnetic flux coupled through the dielectric window sustains an inductively coupled plasma in chamber gas below the window. A workpiece or substrate for processing is commonly supported on a horizontal substrate holder or chuck in the chamber. The dielectric window can be flat, although dome shaped windows have also be used in conventional ICP processing apparatus.
In many applications, such as plasma etching or plasma assisted chemical vapor deposition for the fabrication of integrated circuits, it can be essential to maintain a relatively uniform plasma over the various areas of a substrate being processed. With regards to uniformity, a flat dielectric window is often preferred to a dome shape, since a flat widow provides relatively uniform distance between various portions of the plasma source and the workpiece on the substrate holder. Since gas pressure in the processing chamber can be substantially below one atmosphere, the top dielectric window must be thick enough to withstand mechanical stress arising from atmospheric pressure. Mechanical considerations require that the minimum thickness of the window is approximately proportionate to the window diameter. Where the chamber diameter is sufficient to process a 300 mm semiconductor wafer (approximately 0.5 m diameter), a planar quartz window must be at least a few cm thick to withstand atmospheric pressure.
When processing a workpiece in conventional ICP equipment, there is generally a relatively large distance between the external applicator and plasma in the chamber. Coupling between the applicator and ICP is relatively weak when the distance from applicator to plasma gas is relatively large. In general, RF power loss is more than proportionate to the applicator voltage. For example, RF power loss can increase in proportion to the square of the voltage applied to an applicator. Since weak coupling requires relatively high applicator voltage to transfer a predetermined amount of power to an ICP, it reduces the RF power transfer efficiency. Furthermore, relatively high power loss in the applicator and/or in the matching network is associated with ICP instability. Low efficiency has also made it difficult or unfeasible to maintain a low power and/or low density plasma in an inductive, rather than a capacitive mode. Hence it has been relatively difficult to perform processing at low power or at low plasma density with an ICP. Furthermore, ICP equipment has been burdened with costs of excess power supply capacity and the necessity of removing heat produced in power losses.
Plasma uniformity control is also relatively difficult where there is a substantial separation between the applicator and plasma. In principle, spatial plasma uniformity in the chamber might be improved by using a plurality of applicator coils and directing various amounts of power into coils at different positions adjacent to the window. However there is generally poor spatial correlation between coil current and the adjacent plasma density related to substantial separation between the coils over a window and process gas interior to the chamber. Hence this technique has been relatively ineffective.
Nonuniformity can also arise through nonuniform feed gas introduction. In some capacitive plasma processing equipment, an applicator electrode above a workpiece support has “showerhead” gas distribution holes for selectively introducing feed gas in a uniform manner. Generally, in ICP processing apparatus having an external inductive applicator above a workpiece, a large and relatively thick flat or dome-shaped dielectric window has been necessary to support external atmospheric pressure and allow magnetic flux into the chamber for powering the plasma. Such windows are have often been made from quartz or a ceramic. It has been impractical to introduce feed gas through large thick windows owing to structural/mechanical limitations and/or cost. Hence feed gas has been introduced into these chambers in a different manner. For example, in some ICP processing apparatus, feed gas has been introduced into the processing chamber through a plurality of feed injectors at various positions around the periphery of the substrate and/or below the substrate holder. However it has been relatively difficult to effect uniform gas distribution over the substrate using such means and injectors in the chamber can have adverse effects on plasma uniformity.
It can be seen that there is a need for efficient ICP processing methods having relatively higher coupling between the applicator and plasma. There is also a need for ICP processing with improved power transfer efficiency and uniformity. Furthermore, there is a need for ICP processing methods that are operably stable at low power and/or low plasma density. Still further there is a need for ICP processing methods and apparatus with improved feed gas distribution.